The present invention relates to a decoding device and a decoding method and, more particularly, to a decoding device and a decoding method which improve the quality of pictures at inverse playback when moving pictures stored in a storage memory are reproduced.
Recently, with the development of the information society, there is a growing request to transmit moving pictures to others beyond the walls of time and distance. An era when the digital technology is put into practical use in full scale has opened, and thus moving pictures have been enabled to be recorded/ reproduced by a recording device or transmitted by long distance using a communication network. Further, not only in the field of communication, but also in the field of broadcasting, a transmission and coding method using the digital technology is adopted.
Since digital signals of a moving picture or sound signals generally have a large amount of codes, the utilization of high-efficiency coding technology is considered to be indispensable to efficiently record and transmit the signals, and coding devices and decoding devices have already been put into practical use.
A method based on a method which is described in International Standard System xe2x80x9cInformation Technologyxe2x80x94Coding of moving pictures and associated audio for digital storage media at up to about 1.5 Mbits/s (ISO/IEC11172-2)xe2x80x9d, commonly called MPEG (Moving Picture Experts Group) System, is sometimes used as the above-described, high-efficiency coding technology.
A video CD which is a CD (compact disk) having digital moving pictures recorded thereon or a DVD having digital moving pictures of higher quality and longer time than those recorded on a video CD, which are recorded thereon, is one of the applications.
In a decoding device using a storage media, like a player for these video CDs or DVDs, not only the normal playback function for reproducing moving pictures from the past to the future on a time axis, but also the inverse playback function for reproducing moving pictures from the future to the past are essential.
Hereinafter, a prior art decoding method utilizing the method described in MPEG will be explained with reference to drawings.
Initially, a method for coding a digital moving picture in a MPEG system and a bitstream therein will be described.
In a MPEG system, it is assumed that a digital moving picture is constituted by a series of video frames, and a series of video frame groups which is referred to as a sequence comprising a plurality of the video frames is coded. The sequence is divided into a series of video frame groups, each normally having a time length of about 0.5 sec. to be subjected to coding, and each of which is referred to as a Group of Pictures (hereinafter, referred to as a GOP).
The GOP is constituted by I (intra) coded frames, which I coded frame is coded by using only data of the video frame by itself, P (predictive) coded frames, which P coded frame can be prediction-coded with reference to data of temporally previous frames (I coded frames and P coded frames), and B (bi-directionally predictive) coded frames, which B coded frame can be bi-directionally prediction-coded with reference to I coded frames and P coded frames which are temporally previous or subsequent.
Next, a method of inverse playback in a MPEG bitstream will be described.
At the normal playback, all pictures in a bitstream are decoded and displayed. On the other hand, at the inverse playback, a bitstream of each GOP is repeatedly transferred to a decoding device, temporally going back from the future to the past, and only the first frame included in each GOP is successively decoded and displayed. These situations are shown in FIGS. 13(a) and (b). FIG. 13(a) illustrates output pictures at the normal playback, showing all frames included in a stream. Here, GOP(n) represents an n-th GOP from the head of a sequence. At the inverse playback, the streams of GOPs are transferred in a descending order of, like GOP (n+1), GOP(n), GOP (nxe2x88x921) and only the first I coded frames (12 coded frames, respectively) included in the respective GOPs are decoded and displayed (see FIG. 13(b)).
The prior art decoding method is constituted as described above, and at the inverse playback, as shown in FIG. 13(a), a reproduced picture is constituted by images at an interval of 6 frames or 12 frames, depending on the numbers of frames in GOPs included in supplied bitstreams, and results in dispersion. Consequently, a smooth reproduced picture cannot be obtained. Particularly when reproducing a stream which is optimized to be coded with an I coded frame which can be coded with high quality when a scene change is generated where there is a great possibility of the lengths of GOPs varying, in order to obtain high quality of pictures, the numbers of frames included in GOPs are variable, whereby a smooth reproduced picture cannot be obtained.
It is an object of the present invention to provide a decoding device and a decoding method which can realize a smooth reproduced picture without losing naturality in the reproduced picture at the inverse playback.
Other objects and advantages of the present invention will become apparent from the detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the spirit and scope of the invention will be apparent to those of skill in the art from the detailed description.
According to a 1st aspect of the present invention, a decoding device which divides a moving picture into a plurality of continuous picture groups, receives a code sequence which is obtained by performing coding processing to each of the picture groups as an input, and decodes independently coded frames which are coded independently within pictures, and predictively coded frames which are coded with reference to other frames of different times, to obtain a decoded picture, comprises: means for setting a frame interval for selecting frames at a prescribed interval from continuous frames; means for deciding, with a first frame which is presently displayed in the continuous frames as a reference point, a frame which is to be decoded later, on the basis of the frame interval set by the frame interval setting means; and decoding means for decoding a code sequence corresponding to the frame decided by the picture output candidate frame deciding means. Therefore, frames which are selected at a uniform playback interval on a time axis at the normal playback are decided as picture output candidate frames and thereby the number of picture output frames increases, whereby a smooth reproduced picture is obtained.
According to a 2nd aspect of the present invention, in the decoding device of the 1st aspect, when the number of frames in a second picture group preceding a first picture group including the first frame is unknown, the picture output candidate frame deciding means analyze types of coded frames and the numbers of respective type coded frames, which are included in the code sequence, to determine the number of the frames, and thereafter decide the picture output candidate frame. Therefore, the same effects as those of the 1st aspect are obtained.
According to a 3rd aspect of the present invention, in the decoding device of the 1st aspect, when the number of frames in a second picture group preceding a first picture group including the first frame is unknown, the picture output candidate frame deciding means decide a predictively coded frame or an independently coded frame, which is located last in the second picture group, as the picture output candidate frame. Accordingly, when the number of frames in a preceding picture group is unknown, a code sequence of the preceding picture group is analyzed to determine the number of the frames, and a frame which is most recently decoded among two frames of a predictively coded frame and an independently coded frame stored in frame memories at this time is decided as a picture output candidate frame. Therefore, the reading operation for determining the number of frames and the decoding operation can be performed simultaneously, whereby higher-speed inverse playback is realized.
According to a 4th aspect of the present invention, in the decoding device of the 1st aspect, when the number of frames in a second picture group preceding a first picture group including the first frame is unknown, the picture output candidate frame deciding means decide a predictively coded frame or an independently coded frame, which is located last but one in the second picture group, as the picture output candidate frame. Therefore, the same effects as those of the 3rd aspect are obtained.
According to a 5th aspect of the present invention, in the decoding device of the 1st aspect, when the number of frames in a second picture group preceding a first picture group including the first frame is unknown, the picture output candidate frame deciding means analyze the code sequence to determine the number of the frames in the second picture group and thereafter decide, as the picture output candidate frame, a frame which is nearer to the picture output candidate frame, among either a last predictively coded frame or a last independently coded frame in the second picture group, or a last but one predictively coded frame or a last but one independently coded frame in the second picture group. Therefore, the same effects as those of the 3rd aspect are obtained.
According to a 6th aspect of the present invention, in the decoding device of any of the 2nd to 5th aspects, when the decided picture output candidate frame cannot be completely decoded with only a code sequence corresponding to the second picture group, the picture output candidate frame deciding means decide a first independently coded frame included in the second picture group as the picture output candidate frame. Therefore, the same effects as those of the 3rd aspect are obtained.
According to a 7th aspect of the present invention, a decoding method which divides a moving picture into a plurality of continuous picture groups, receives a code sequence which is obtained by performing coding processing to each of the picture groups as an input, and decodes independently coded frames which are coded independently within pictures, and predictively coded frames which are coded with reference to other frames of different times, to obtain a decoded picture, comprises: a step of setting a frame interval for selecting frames at a prescribed interval from continuous frames; a step of deciding, with a first frame which is presently displayed in the continuous frames as a reference point, a frame which is to be decoded later, on the basis of the set frame interval; and a decoding step of decoding a code sequence corresponding to the frame decided in the picture output candidate frame deciding step. Therefore, frames which are selected at a uniform playback interval on a time axis at the normal playback are decided as picture output candidate frames and thereby the number of picture output frames increases, whereby a smooth reproduced picture is obtained.
According to an 8th aspect of the present invention, in the decoding method of the 7th aspect, the picture output candidate frame deciding step comprises: a step of judging whether the number of frames in a second picture group preceding a first picture group including the fist frame is unknown or not; a step of, when it is judged in said number of frames judging step that the number of the frames is unknown, analyzing types of coded frames and the numbers of respective type coded frames, which are included in the code sequence, to determine the number of the frames. Therefore, the same effects as those of the 7th aspect are obtained.
According to a 9th aspect of the present invention, in the decoding method of the 7th aspect, the picture output candidate frame deciding step comprises: a step of judging whether the number of frames in a second picture group preceding a first picture group including the first frame is unknown or not; and a step of, when it is judged in the number of frames judging step that the number of the frames is unknown, deciding a predictively coded frame or an independently coded frame, which is located last in the second picture group, as the picture output candidate frame. Accordingly, when the number of frames in a preceding picture group is unknown, a code sequence of the preceding picture group is analyzed to determine the number of the frames, and a frame which is most recently decoded among two frames of a predictively coded frame and an independently coded frame stored in frame memories at this time is decided as a picture output candidate frame. Therefore, the reading operation for determining the number of frames and the decoding operation can be performed simultaneously, whereby higher-speed inverse playback is realized.
According to a 10th aspect of the present invention, in the decoding method of 7th aspect, the picture output candidate frame deciding step comprises: a step of judging whether the number of frames in a second picture group preceding a first picture group including the first frame is unknown or not; and a step of, when it is judged in the number of frames judging step that the number of the frames is unknown, deciding a predictively coded frame or an independently coded frame, which is located last but one in the second picture group, as the picture output candidate frame. Therefore, the same effects as those of the 9th aspect are obtained.
According to an 11th aspect of the present invention, in the decoding method of the 7th aspect, the picture output candidate frame deciding step comprises: a step of judging whether the number of frames in a second picture group preceding a first picture group including the first frame is unknown or not; a step of, when it is judged in the number of frames judging step that the number of the frames is unknown, analyzing types of coded frames and the numbers of the respective type coded frames, which are included in the code sequence, to determine the number of the frames; and a step of selecting a frame which is nearer to the picture output candidate frame, among either a last predictively coded frame or a last independently coded frame in the second picture group, or a last but one predictively coded frame or a last but one independently coded frame in the second picture group and deciding the frame as the picture output candidate frame. Therefore, the same effects as those of the 9th aspect are obtained.
According to a 12th aspect of the present invention, in the decoding method of any of the 8th to 11th aspects, the picture output candidate frame deciding step comprises: a step of judging whether the decided picture output candidate frame can be completely decoded with only a code sequence corresponding to the second picture group; and a step of, when it is judged in said decoding state judging step that the decoding cannot be completely performed with only the code sequence corresponding to the second picture group, designating a first independently coded frame included in the second picture group and deciding the frame as the picture output candidate frame. Therefore, the same effects as those of the 9th aspect are obtained.